1. Field of the Invention
The invention relates to a process for roughening aluminum or aluminum alloys as support material for printing plates, in which process two electrochemical roughening steps are carried out in direct succession. The invention also relates to a printing plate comprising a support material which is produced by the process.
2. Description of Related Art
Printing plates, in particular offset printing plates, generally comprise a support and at least one radiation-sensitive coating arranged thereon, said coating being applied to the coating support by the user in the case of non-precoated plates or by the manufacturer in the case of precoated plates.
Aluminum or one of its alloys have found acceptance as coating supports in the printing plate sector. In principle, these coating supports can be used without a modifying pretreatment, but in general they are modified in or on the surface, for example by a mechanical, chemical and/or electrochemical roughening, which is sometimes also termed graining or etching, a chemical or electrochemical oxidation and/or a treatment with agents which render the surface hydrophilic.
In modern continuous high-speed plants for the production of printing plate supports and/or precoated printing plates a combination of the said processing steps is frequently used, in particular a combination of electrochemical roughening and anodic oxidation, optionally with a subsequent step for rendering the surface hydrophilic.
The roughening can be carried out in aqueous acids, for example aqueous HCl or HNO.sub.3 solutions, or in aqueous salt solutions, for example aqueous NaCl or Al(NO.sub.3).sub.3 solutions, applying alternating current. The peak-to-valley heights of the roughened surface which are achievable in this way and which are given, for example, as average peak-to-valley heights R.sub.z are in the range from 1 to 15 .mu.m, in particular in the range from 2 to 8 .mu.m. The peak-to-valley height is determined in accordance with DIN 4768 in the October 1970 version. The arithmetic mean of the individual peak-to-valley heights of five adjacent individual measured sections is calculated as the average peak-to-valley height R.sub.z.
The roughening is carried out, inter alia, in order to improve the adhesion of the reproduction coating to the coating support and of the damping agent supply to the printing form formed from the printing plate by exposure and development.
The water supply is an important quality characteristic for offset printing plates. It is defined in the publication "Ermittlung einer optimalen Wasserfuhrung zur Steigerung der Leistungsfahigkeit des Offsetdruckes" [Determination of an optimum water supply to increase the performance of offset printing] (Albrecht, J.; Rebner, W., Wirz, B., Westdeutscher Verlag, Cologne and Opladen 1966, page 7) as the metering and control of the damping of the printing form during the printing run. The water supply also depends, inter alia, on the surface roughness of the printing form, i.e., graining of the surface. The problems of inadequate water supply are adequately known: if too much water is required to keep non-printing parts of a printing form free from ink, more water is able to emulsify into the ink and the print becomes flat. Moreover, water marks can be produced, the paper becoming damp. In addition, register problems can arise and in the case of web-offset printing there is an increased risk of the paper web tearing. The above lists only a few of the problems. Comments on the significance of a correct water supply can also be found in the publication "Beitrag zur Analyse des Offsetprozesses" ("Contribution on the analysis of the offset process"), pages 17-18 (Deoker, P.; Polygraph Verlag, Frankfurt am Main). In this publication the consequences of too high and too low damping agents supply are discussed. This term is more appropriate than the term "water supply" in so far as, in offset printing, in general, pure water is not used for damping, but usually several components are added to the water.
In the cited publication, the disadvantages of an excessive damping agent supply, which have already been mentioned above, are listed. However, too low a supply of damping agent is also a disadvantage. If the printing plate in the printing machine is supplied with too little damping agent, as a result of too low a setting of the damping unit, or if the printing plate requires more damping agent than the damping unit of the printing machine is able to supply by reason of its construction or on other grounds, parts of the printing plate which otherwise are non-printing are also able to take up ink and co-print, fine raster areas being particularly sensitive to co-printing. The co-printing of non-image areas within the raster areas is known as "smearing in".
Thus, a worthwhile aim is a printing plate which requires only very little damping agent, in order to still keep fine rasters, but also large-area non-image areas, free from ink, but which, on the other hand, also shows a neutral reaction towards large amounts of damping agent and gives flawless prints even if the damping agent supply at times exceeds the norm as a result of plant-induced fluctuations.
It is true that the damping agent consumption of a printing plate can be determined objectively with sufficient accuracy, but this is not the case for the damping agent supply, since there are no objective methods of determination for some of the above-mentioned adverse phenomena, for example smearing in (Decker, P., in "Beitrag zur Analyse. . ." ["Contribution on the analyse. . ."], page 18). For this reason the damping agent supply to a printing plate is here assessed qualitatively, using the adjectives "very good", "good", "satisfactory", "adequate", "moderate", "poor" and "very poor". The conditions under which these adjectives form the basis for the assessment are described below in the context of the discussion of the examples.
A further quality characteristic of an offset printing plate is the brightness and the uniformity of the brightness of the support material. The brightness can, for example, be determined in the manner described in DIN Standard 6174 in the January 1979 version. This standard also indicates how the uniformity of the color print can be quantified. In this standard the value .delta.E.sub.ab*, which can be calculated from the three colour values L*, a* and b*, is used as a measure for the uniformity. A support must not be too dark, so that not too much of the incident light is absorbed by the support surface itself and is thus lost to photochemical reactions in the actual light-sensitive coating. Similarly, the surface should be uniformly bright, so that the sensitivity to light does not vary from location to location on the printing plate.
By means of the exposure or irradiation and development or decoating in the case of process coatings which act electrophotographically, the image areas, which carry ink during subsequent printing, and the non-image areas, which carry damping agent and which generally are composed of the exposed support surface, are produced on the printing plate and by this means the actual printing form is formed. Very diverse parameters have an influence on the subsequent topography and thus on the damping agent supply on the surface to be roughened. For example, the following literature references provide information on this:
In the article "The Alternating Current Etching of Aluminum Lithographic Sheet" by A. J. Dowell in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, the fundamental principles of the roughening of aluminum in aqueous hydrochloric acid solutions are discussed, the following process parameters being varied and the corresponding effects are studied. In the case of repeated use of the electrolyte, the electrolyte composition is changed, for example in respect of the H.sup.+ (H.sub.3 O.sup.+) ion concentration, which can be determined via the pH value, and the Al.sup.3+ ion concentration, with observable effects on the surface topography. Temperature variation between 16.degree. C. and 90.degree. C. shows a modifying influence only above about 50.degree. C., which is discernable, for example, in the substantial decline in coating formation on the surface. The roughening period, of between 2 and 25 min, also leads to an increasing dissolution of metal with increasing period of action. Variation in the current density between 2 and 8 A/dm.sup.2 also results in higher roughness values with increasing current density. If the acid concentration is in the range of 0.5 and 2% HCl, only minor changes in the hole structure occur, below 0.5% HCl there is only a local attack at the surface and at high values an irregular dissolution of aluminum occurs. If pulsed direct current is used instead of alternating current, it is found that both half-wave types are apparently required for a uniform roughening. In this literature reference it is pointed out that the addition of sulfate ions increasingly leads to undesired, coarse, non-homogeneous roughening structures, which are not suitable for lithographic purposes.
The establishment of a flat and uniform surface topography is difficult in pure hydrochloric acid electrolytes and in this case it is necessary to keep the operating conditions within very narrow limits.
The influence of the composition of the electrolyte on the roughening quality is also described, for example, in the following publications:
DE-A 22 50 275 (=GB-A 1,400,918) names aqueous solutions containing 1.2 to 1.5% by weight of HNO.sub.3 or 0 4 to 0.6% by weight of HCl and optionally 0.4 to 0.6% by weight of H.sub.3 PC.sub.4 as electrolytes for the alternating current roughening of aluminum for printing plate supports,
DE-A 28 10 308 (=U.S. Pat. No. 4,072,589) names aqueous solutions containing 0.2 to 1.0% by weight of HCl and 0.8 to 6.0% by weight of HNO.sub.3 as electrolytes for the alternating current roughening of aluminum.
The purpose of additives to HCl electrolytes is to prevent adverse local attack in the form of deep holes. Thus, the following additions are described:
monocarboxylic acids, for example acetic acid, in DE-A 28 16 307 (=U.S. Pat. No. 4,172,772),
gluconic acid, in U.S. Pat. No. 3,963,594,
citric acid and malonic acid, in EP-A 0,036,672 and
tartaric acid, in U.S. Pat. No. 4,052,275.
All of these organic electrolyte constituents have the disadvantage that they become electrochemically unstable and decompose at high current load, which is to be equated with high voltage load.
DE-A 35 03 927 describes ammonium chloride as an inorganic additive to a HCl electrolyte.
Inhibiting additives, as described as phosphoric acid or chromic acid in U.S. Pat. No. 3,887,447 and as boric acid in DE-A 25 35 142 (=U.S. Pat. No. 3,980,539), have the disadvantage that the protective effect frequently collapses locally and individual, particularly pronounced graining is able to form in the affected areas.
JP-A 91 334/78 discloses an alternating current roughening in an electrolyte composed of hydrochloric acid and an alkali metal halide for the production of a lithographic support material.
DE-A 16 21 115 (=U.S. Pat. No. 3,632,486 and U.S. Pat. No. 3,766,043) mentions a direct current roughening in dilute hydrofluoric acid, the aluminum strip being connected as the cathode.
Another known possibility for improving the uniformity is the modification of the type of current used. These include, for example,
alternating current, with which the anode voltage and the anodic coulomb input are greater than the cathode voltage and the cathodic coulomb input (DE-A 26 50 762=U.S. Pat. No. 4,087,341), the anodic alternation time of the alternating current generally being set at less than the cathodic alternation time; reference is also made to this method, for example, in DE-A 29 12 060 (=U.S. Pat. No. 4,301,229), DE-A 30 12 135 (=GB-A 2,047,274) or DE-A 30 30 815 (=U.S. Pat. No. 4,272,342),
alternating current, with which the anode voltage is clearly increased compared with the cathode voltage (DE-A 14 46 026 =U.S. Pat. No. 3,193,485), and
interruption of the current flow for 10 to 120 s, and current flow for 30 to 300 s, alternating current and, as electrolyte, an aqueous 0.75 to 2 N HCl solution containing added NaCl or MgCl.sub.2 being used (GB-A 879,768). A similar process with interruption of the current flow in the anode or cathode phase is also described in DE-A 30 20 420 (=U.S. Pat. No. 4,294,672).
The said methods give aluminum surfaces which, it is true, have a relatively uniform hole size distribution, but require relatively high expenditure on apparatus and can also be used only within very narrow parameter limits. Moreover, the supports can be produced with uniform brightness only with difficulty.
Another procedure disclosed in the patent literature is the combination of two roughening processes. Compared with the one-step process, this has the advantage that, depending on the process control, the influence of one or the other step can predominate within certain limits predetermined by the characteristics of the individual steps.
U.S. Pat. No. 3,929,591, GB-A 1,582,620, JP-A 123 204/78, DE-A 30 31 764 (=GB-A 2,058,136), DE-A 30 36 174 (=GB-A 2,060,923), EP-A 0,131,926, DE-A 30 12 135 (=GB-A 2,047,274) and JP-B 16 918/82 describe the combination of a prestructuring, carried out mechanically in the first step, followed by an optional chemical cleaning (pickling), with an electrochemical roughening by means of modified alternating current in electrolytes containing hydrochloric acid or nitric acid, it being possible for a further cleaning step then to take place.
These processes make use of the advantage of double roughening, with a mechanical roughening as the first step, as a result of which, in particular, a current saving is achieved.
DE-A 38 36 810 discloses a double roughening with two electrochemical roughening steps and an etching treatment which takes place between the two roughening steps.
Various two-step processes are known for the production of capacitors from aluminum foils. U.S. Pat. No. 4,525,249 describes a process which uses hydrochloric acid in the first step and in the second step treats the aluminum foil with a dilute nitric acid, which also contains aluminum in the form of aluminum nitrate, in the absence of current. This process does not yield surfaces which are able to meet the current stringent requirements in respect of offset printing plates.
Two-step processes which use electrochemical processes in both steps have also been disclosed. In the process according to U.S. Pat. No. 4,721,552, the first electrolyte contains hydrochloric acid while the second electrolyte can also contain hydrochloric acid in addition to nitric acid. A similar process is described in JP-A 86/051 396. These known processes do indeed give surfaces which are usable for lithographic purposes, but in respect of the fineness of the surface structure, these surfaces are inferior to those which are achieved in accordance with the teaching of DE-A 37 17 654.
U.S. Pat. No. 4,437,955discloses a two-step electrochemical roughening process for the production of capacitors using a hydrochloric acid-containing electrolyte in the first step and a chloride and sulfate ion-containing electrolyte in the second step. The electrolyte in the second step is not acid and in this step the process is carried out using direct current.
A further, two-step, electrochemical process for the production of a capacitor foil is described in U.S. Pat. No. 4,518,471. In this process the electrolytes in both baths are identical and contain dilute hydrochloric acid and aluminum ions. The baths are operated at different temperatures, specifically at 70.degree. to 85.degree. C. in the first step and at 75.degree. to 90.degree. C. in the second step.
The surfaces produced by the latter two processes, which have been optimized for electrolyte capacitors, are too pitted for use in lithography.
DE-A 38 36 810 describes a process in which aluminum is roughened, likewise in two steps, for the production of printing plate supports. In this process pickling is carried out between the first and the second roughening step. This process has the disadvantage that the plates develop an irregular surface and become very dark, especially if chloride-containing electrolytes are used in the final pickling step.